The present invention relates to fluid temperature control apparatus. In particular it is concerned with apparatus for providing fluid, for example water, to a user at a required temperature where this is achieved by mixing in varying proportions two supplies of liquid at different temperature, designated "cold" and "hot".
The apparatus is of general application but is best explained in relation to control of a standard North American domestic hot water tank.
In a conventional water heating systems, cold water, delivered to a combined water heater/storage tank, is heated to a desired temperature in readiness for demand draw by the user.
In a direct demand system, which is commonly used for domestic hot water supply, the apprehension of scalding the user, generally results in fixing the maximum temperature to which water may be heated in the water heater tank, to a relatively low setting such as 60 degree C. (140 degree F.) or below.
However, such a tepid water temperature can encourage bacterial growth in the water within the storage tank. It is therefore preferred, for sanitary purposes, that the water in the tank be heated to a sanitizing temperature to destroy bacteria or other potential growth organisms.
Trihey, John M. of Australia, (PCT patent number PCT/AU90/00159) teaches a method of tempering hot water by creating a flow path between a cold water inlet and a hot water outlet. The flow path is created using a tube of relatively narrow bore. A manual valve is disposed within the tube and can be set to control the temperature of the water in the distribution system. The ratio of cold water and hot water mix is determined by the size of the tube and the setting of the manual valve. Unfortunately, this predetermined ratio does not take into account the varying temperature of the cold water supply, the changes in pressure within the hot water tank as the temperature of the water therein increases or decreases, and decreases in the water temperature as water is drawn from the hot water tank. As a result, since the ratio is predetermined, the user does not get the benefit of the tepid water left in the tank, since the tepid water will be further cooled by mixing it with water from the cold water inlet.
Perlman et al. of Canada (U.S. Pat. No. 5,115,491) teaches a tempering system using a heat exchanger and a valve. To get maximum benefit from the system the water within the storage tank is heated to scalding temparature. However, the heat exchanger makes the system too expensive. Furthermore, the use of a heat exchanger makes the system prone to scaling problems.
The present invention overcomes these disadvantages by monitoring the temperature of water in the hot water distribution system and adjusting, in real time, the ratio of cold water mixing with hot water from the tank.
In a preferred embodiment of the invention the ratio of the cold water and hot water mixing is further adjusted by the temperature of water at the hot water outlet and also by the temperature of the cold water supply. As will be appreciated, the present system takes seasonal changes in the temperature of cold water supply as well as changes in temperature of hot water in the tank due to heating elements turning on or due to water being drawn from the hot water tank by the user, into account.
In a preferred embodiment of the present invention, a stepper motor valve which controls the ratio of the cold water mixing with hot water is installed near the cold water inlet. This position has a number of advantages as discussed below:
1. The cold water supply reduces the temperature around the valve, which can damage the electronic components. PA1 2. The scaling problem within the valve is mitigated as scaling typically occurs on the hot water outlet side. PA1 3. The the pressure within the tank is stabilized and so hot water, from the hot water outlet does not reach the valve thereby to inhibit the hot water slug.
The present invention further provides a means of preventing a slug within the fluid tempering system. Slug is the hot fluid left in the tempering system after the consumer has turned off the flow of fluid or the tempering system is not used for a period of time. The slug is created by hot fluid in the tank being at a higher temperature and therefore at a higher pressure. The slug pushes itself into the hot fluid distribution system. The temperature of the slug can be at a high enough temperature to cause scalding problems.